The majority of printers are 1-bit devices. This means they only have one shade of each primary colours—namely, Cyan, Magenta, Yellow and Black. (Black is normally not included as it helps on shadows but is not really a colour.) These devices can either put down the dot—a Cyan, Magenta or Yellow (CMY) dot—or not. By combining (overlaying) the CMY dots, a total of 7 colours can be created (23−1, since the absence of all three colours is no colour).
In order to create the effect of thousands or millions of colours, a process called screening is used, which arranges the dots to create the effect of more colours. The higher the resolution of the device, the more effective the screening is and, therefore, the more colours can be created. There are many different types of screening. Some examples are Error diffusion, Stochastic/Thresholds, and PostScript halftones.
Different devices require different types of screening in order to get the best output. For example, error diffusion and stochastic screens look great on inkjets, but the same screen used on thermal transfer printers produce horrible artefacts in the midtones. For this reason, it is more common to use traditional halftones on thermal transfer printers.
A halftone cell is a square group of pixels. To generate a bi-tonal halftone cell for a black-and-white output device (two tones: black and white), each pixel in the halftone cell is turned ON or OFF so the outcome will be a halftone cell with some pixels ON (black dot) and others OFF (white or blank dot). By relying on the fact that an observer's eye will spatially average over the pixel area, the effect of intermediate tone levels (shades of gray) is created.
The same principle is used to generate multi-tonal halftone cells to create the appearance of intermediate colour values. In a multi-tonal halftone cell, each pixel can assume any of the 8 tones, which are created by the overlaying of the CMY dots. (Here, no colour is counted as a tone since a blank dot has an effect on the overall outcome.) When the observer's eye spatially averages over the pixel area, the effect of intermediate color values (colour shades) is created.
The halftone cells, each representing an intermediate tone level (bi-tonal) or color value (multi-tonal), are conceptually tiled across the page so that any address within the page has a corresponding address within the halftone cell. This then acts as a mask controlling how ink is added to the output device. Output produced using this technique, when viewed from a suitable distance, fools the observer into perceiving many more shades than are present in reality.
To increase the number of shades (tone levels or colour values), different dot sizes are used. This is called “variable-dot halftoning.” The variable-dot halftoning could be used with black-and-white or colour output devices. However, for Simplicity, it will be explained with respect to the black-and-white output devices, which use bi-tonal halftone cells.
A black-and-white variable-dot output device, in addition to having the capability of turning a pixel within its bi-tonal halftone cell ON or OFF, has the ability to determine the size of each ON dot. For example, a variable-dot device that has 3 dot sizes has the ability to turn a dot OFF, ON-and-small, ON-and-medium, or ON-and-large. This, of course, substantially increases the number of tone levels that can be obtained. The device described in this example is referred to as a 2-bit device-meaning, 2 bits per pixel, the 2 bits representing the 4 (22) different states that a pixel can assume.
It should be easy for someone skilled in the art to see how the same principle could be applied to the colour output devices to extensively increase the number of colour values that can be obtained. A 3-bit device, for example, could be used to implement the overlaying of the CMY colours as discussed above. A pixel with 3 bits can assume up to 8 states (23).
A halftone is controlled by three parameters: frequency, angle and spot function (dot shape). Frequency (often referred to as LPI for Lines Per Inch) is the number of halftone cells per inch. For example, a frequency of 60 would give 60 cells every inch.
The size of a cell is determined by the frequency. At a resolution of 300 dpi (dots per inch), a frequency of 60 LPI would give a cell size of 5×5 pixels (300/60). At the same resolution, a frequency of 100 LPI would give a cell size of 3×3 pixels (300/100).
The number of colour shades that can be printed is determined by the size of the cell. A 5×5 cell can display 25 shades, and from this one can work out the total number of colours that can be produced. For a CMY device, this would be 25×25×25 (253).